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i think the main problem is you guys think your options are limited to a smart or not so smart little white box

a canary or similar, can be used for O detection, you can even recycle the battery detection spring from your old sensor housing, weight detection plate..

for about 25p a day, a cat is a great fire detector, i would sugest one for each room..

this also solves several other problems of xmas/birthday presents for kids, who will push their test buttons daily, be much happier and more interested in fire prevention etc